This invention relates to a reel shaft device of a magnetic recording tape running apparatus.
Generally, known is a reel shaft device which comprises a reel hub engaging member engaging a reel hub to rotate therewith, a rotating member capable of rotating relatively to the reel hub engaging member, a driving member having a circumferential surface facing a circumferential surface of the rotating member and capable of rotating concentrically with the reel hub engaging member, the driving member being rotated by a rotary power supply means, a plurality of rolling members disposed between the circumferential surface of the rotating member and the circumferential surface of the driving member so as to be able to rotate on their own axes, a cam member disposed on one of the respective circumferential surfaces of the rotating member and the driving member, the cam member holding the plurality of rolling members in cooperation with the other of the respective circumferential surfaces of the rotating member and the driving member to rotate the rotating member together wih the driving member in one direction when the driving member rotates in such one direction, and releasing the hold of the plurality of rolling members to allow the rotating member to rotate independently of the driving member when the driving member rotates in the other direction, and a variable rotation transmitting means capable of transmitting rotation between the reel hub engaging member and the rotating member, whereby the rotation transmissibility between the reel hub engaging member and the rotating member is increased substantially in inverse proportion to the decrease of the magnitude of a torque applied to the reel hub engaging member.
The reel hub engaging member and the driving member are concentrically rotatably mounted on a center shaft fixed on the chassis of the magnetic recording tape running apparatus. Further, the rotating member is concentrically rotatably mounted on the outer circumferential surface of the reel hub engaging member, and a gap with a minimum necessary size to allow the rotation of the rotating member is defined between the rotating member and the outer circumferential surface of the reel hub engaging member. Accordingly, the conventional rotating member cannot rotate as it is eccentric with respect to the outer circumferential surface of the reel hub engaging member.
With the prior art reel shaft device of such construction, if the outer circumferential surface of the reel hub engaging member is eccentric with respect to the center shaft for some reason such as manufacturing error, some of the plurality of rolling members will fail to be held by the other of the respective circumferential surfaces of the rotating member and the driving member when the driving member rotates in the one direction. If all of the rolling members are not held by the circumferential surface of the rotating member or the driving member and the cam member, the torque of the rotatory power transmitted from the driving member to the rotating member will vary, so that the reel hub engaging member and the reel hub will be subject to irregularity in rotation. Such irregularity in rotation of the reel hub will cause wow or flutter.